1. Field of the Invention
The present invention generally relates to a signal line switching circuit. More particularly, this invention relates to a circuit for performing a switching operation of a protection line between a common section and a channel section.
2. Description of the Prior Art
In a communication system, there are many structural arrangements for connecting a channel section having an interface function with a lower level system to a common section having a function for monitoring the entire system, an interface function with the higher level monitoring system or an interface function with other equipments via a signal line.
These structural arrangements generally function to bypass a disabled communication by switching the signal line to a protection line when a communication fault occurs to the interface functions. The conventional structural arrangements illustrated in FIG. 4, for example, function in this manner. It is noted that in FIG. 4(a), reference numeral "1" is the common section (CMS), whereas numeral "2" is the channel section (TRIB).
In the case where multiple channels (CH) are connected to the common section (CMS), interface circuits 21, 22, 23 corresponding to multiple channels (CH) are provided on the channel section (TRIB) 2. Similarly, interface circuits 11, 12 are provided on the common section (CMS) 1, corresponding to the interface circuits 21, 22 located on the channel sections (TRIB) 2.
Additionally, in FIG. 4, two interface circuits 21, 22 are shown representative of the multiple interface circuits on the channel section (TRIB) 2. Further, the interface circuits 11, 12 on the common section (CMS) 1, which are provided corresponding to two interface circuits 21, 22 on the channel section (TRIB) 1, are shown.
Furthermore, each of the interface circuits 11, 12, 21, 22 of the common section (CMS) 1 and the channel section (TRIB) 2 has a circuit for protection (P), as well as a working circuit (W), to prevent the occurrence of a communication disability by switching from the working circuit to the protecting circuit when the disconnection of the signal is detected. Consequently, in the conventional device illustrated in FIG. 4, the disconnection signal is detected by either a clock 31 outputted from a clock extracting circuit 3 from a data-receiving section (receipt data section) or a clock 32 prepared at the receipt side synchronizing with the received data in the data-receiving section.
That is, a counter 4 for counting signal 5, shown in FIG. 4, is further provided, wherein the counter 4 is reset with the clock 31 or the clock 32. Accordingly, the counter 4 continues to count without being reset when the signal disconnection is continued. The alarm is subsequently generated; and the signal disconnection is detected when the counted value reaches a predetermined value.
With the above-discussed conventional structural arrangement, when multiple channel sections (TRIB) are connected to the common section (CMS), the number of interface circuits becomes too large; i.e., the channel number to be handled becomes too large so that the disconnection signals to be detected and the interface circuits for the protection lines become unnecessarily numerous.
Moreover, in the above-discussed conventional structural arrangement, as the disconnection signal is detected by using the clock, the circuit for detecting disconnection has to comprise many parts; thereby, resulting in the use of a large mounting area, the increase in cost due to numerous needed parts, and the increase in power supply consumption.
It is therefore an object of the present invention to provide a signal line switching circuit for solving the abovediscussed problems of the conventional structural arrangement.